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Cortina GT tach

 
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mortarch



Joined: 27 Feb 2009
Posts: 12
Location: knoxville

PostPosted: Sun May 31, 2009 8:22 am    Post subject: Cortina GT tach Reply with quote

Group,
The tach in my '68 Cortina GT indicates twice the actual engine speed. Thoughts? Solutions?
Thanks,
mortarch
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angliagt



Joined: 11 Jun 2007
Posts: 693
Location: Eureka,California

PostPosted: Sun May 31, 2009 3:56 pm    Post subject: Reply with quote

Possibly a MKI tach?They're positive ground,
MKII's are negative ground.Just a wild guess.

- Doug
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peteleo



Joined: 24 Aug 2008
Posts: 878
Location: San Mateo,California

PostPosted: Sun May 31, 2009 9:12 pm    Post subject: Tach Reply with quote

Remove the tach from the shroud.

Disconnect the wires to the tach.

Twist the chrome bezel by hand to unlock and remove. That may be hard to do so lift the tabs slightly to ease movement.

Remove the two or three slotted screws from the back of the tach and remove base cover.

Locate the largest resister on the circuit board ( 150 ohm 1/2 watt - colors: brown/yellow/black ).
Apply an ohm meter across the resistor to check the value. If > 200 ohms relpace it.
If ok, then locate a pot that has a slotted adj. screw on top.
Hook up the bare tach to the dash wiring ( lay it on the column under a rag for support )
Connect a timing light or such with an rpm reading to the engine. Start the engine.
Compare the rpm readings from the tach and T/L. Adjust the pot to the reading on the T/L.

Before attaching the casing to the tach. Drill a 1/4" hole in the casing that lines up to the pot.
That way you can adj. the tach with a screw driver without taking it apart.

Most tachs ( positive or negative grd ) will need adjusting because of age, unless it's new.

Cheers,

Pete
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stanchfi
Site Admin


Joined: 15 Mar 2007
Posts: 842
Location: Indiana

PostPosted: Mon Jun 08, 2009 9:02 am    Post subject: Reply with quote

"twice the actual engine speed"?......um....divide by 2

-James
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mortarch



Joined: 27 Feb 2009
Posts: 12
Location: knoxville

PostPosted: Mon Jun 08, 2009 2:22 pm    Post subject: tach speed Reply with quote

Thanks guys for your responses. I'm pretty sure it is the original negative ground tach, so I'll try the adjustment technique suggested.
Thanks again,
mortarch

ps. The dividing by two works until about 3500rpm. Just makes me really nervous to see the tach reading 7000rpm on a stock crossflow! Smile
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RS.Escort



Joined: 23 Sep 2008
Posts: 74
Location: Vancouver Canada

PostPosted: Sat Jun 20, 2009 1:43 pm    Post subject: Reply with quote

Hey, maybe you have my problem!

There are no sources for switched power accessories under the hood of my Cortina.. that is every wire is only live when the acessory is on, such as lights, signals, horn etc, so when I was installing my electric fan I used the coil positive wire..

Perfect, switches the fan on and off with the key and also a nice place to get the power from as far as accessibility..

My tach also read about 7 grand at idle, but since I put the tach in after the fan I never made the connection that it was picking up the fan motor, as well as the coil.. you know my fan turns at about 7 grand?

Duh.. took me a long time to figure it out, but now I have my fan connected to my park lamps, so I need to drive with my park lamps on all the time, which is not a bad thing safety wise, and now that the fan is off the coil circuit, the tach works perfectly..

Just a thought! Mark
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